As the demand for electric power increases, new power sources (such as Independent Power Producers (IPPs) and Distributed Generators (DGs)) are being added to the power grid. The consequence of this trend is that the fault current in the power system is being increased, exceeding the ratings of existing protection devices, e.g., circuit breakers, relays and fuses.
The IPPs, installed in parallel with existing generators, decrease the equivalent source impedance of the grid. Furthermore, the DGs are usually placed close to the load and the fault. Therefore, the impedance from the DGs to fault is also decreased. The effect of smaller impedance in the network is that the fault current might rise above the rated limits of the existing protection devices. In order to protect equipment (e.g., transformers and electric machines) it becomes necessary to either upgrade the protection devices or lower the threshold fault current level. However, device upgrade is generally not economically viable, considering the enormous number of devices that need replacement. Accordingly, fault current limiters (FCLs) have become a preferred option to address the over-rating issue.
Fault current limiter technology offers several advantages, including, but not limited to, the following: (i) mitigating the effect of high threshold fault current levels on a distribution system, thereby permitting use of lower rated protection devices and deferring costly device replacements; (ii) protecting existing devices from the first large peak during a fault condition, since many FCLs can limit the fault current within the first quarter-cycle; (iii) reducing voltage dips; and (iv) enhancing grid stability.
Traditionally, FCLs have been applied in power generation, transmission and distribution, and in all ranges of voltage levels from 400V to 132 kV. Typical applications of FCLs include busbar coupling and transformer feeder, distributed generation coupling, power plant auxiliaries, and ship propulsion systems.
An FCL ideally possesses the following characteristics: (a) virtually zero impedance under normal operation; (b) fast detection and fast action (e.g., detection of fault current within the first cycle); (c) minimum impact on existing protection relays and circuit breakers; (d) minimum impact on voltage magnitude and phase; and (e) automatic and fast recovery to address repeated faults.
Examples of existing FCL technologies include Superconductor FCL (SCFCL), Solid-State FCL (SSFCL), and Is-limiter. Each of these existing FCL technologies has drawbacks (e.g., high operation losses, bulky size and servicing/part replacement issues). Most SSFCLs and some SCFCLs are subjected to switching losses during normal operation. Moreover, the superconductors in SCFCLs require extra energy to be cryogenized in order to stay in the superconducting state during normal operation. Many FCLs mentioned above are large in dimensions. They either need extra cryogenic equipments, or need large capacitor banks or large iron cores to operate. An Is-limiter is comprised of an extremely fast switch, paralleled with a high rupturing capacity fuse. In order to achieve the desired short opening time, the Is-limiter fires a pyrotechnic charge to open the main conductor. The fault current is then carried by the parallel fuse, which interrupts the fault current at the next zero crossing. The Is-limiter has several drawbacks. For example, the Is-limiter can raise safety concerns because it is not fail-safe, that is, correct operation of the Is-limiter cannot be tested without destroying the Is-limiter (i.e., the parallel fuse). Furthermore, supply of pyrotechnic materials are regulated and constrained by the U.S. Department of Defense, thereby resulting in supply problems and increased manufacturing costs. In addition, replacement of the parallel fuse is required after each triggering, thus leading to high operating cost.
One existing Non-Superconducting FCL (NSCFCL) is a bridge-type non-superconducting FCL proposed by M. T. Hagh and M. Abapour in “Nonsuperconducting Fault Current Limiter With Controlling the Magnitudes of Fault Currents;” Hagh, M. T. and Abapour M.; IEEE TRANSACTIONS ON POWER ELECTRONICS; 2009; Vol. 24; No. 3; pp. 613-619, hereinafter referred to as “Hagh et al.” In Hagh et al.'s NSCFCL, a DC reactor serves two functions: (a) during normal operation, the DC reactor minimizes the current ripple of the rectified DC current; and (b) during a fault condition, the DC reactor is used as impedance during the initial rise of fault current. The rise of the fault current is slowed so that a control circuit has adequate time to operate a semiconductor switch. This NSCFCL has drawbacks that include, but are not limited to: (1) the need for a semiconductor switching unit having a high voltage rating due to the high voltage stress; (2) the need for a discharging resistor having a large resistance; (3) the need for isolation transformers having high voltage ratings; and (4) large DC reactor size with current always through it in either the normal or faulted state. These component requirements result in high manufacturing costs for the NSCFCL.
The present invention provides a non-superconducting fault current limiter that overcomes these and other drawbacks of existing FCLs.